An ophthalmic apparatus includes an ophthalmic measurement apparatus for measuring the properties of a subject's eye and an ophthalmic photographing apparatus for obtaining an image of the subject's eye.
Examples of ophthalmic measurement apparatuses include ocular refraction examination apparatuses (refractometers and keratometers) which measure the refractive properties of subject' eyes, tonometers, specular microscopes for obtaining corneal properties (for example, corneal thicknesses and corneal endothelial cell densities), and wavefront analyzers for obtaining the aberration information of subject's eyes by using Hartmann-Shack sensors.
In addition, examples of ophthalmic photographing apparatuses include optical interferometry tomographic apparatuses for obtaining tomographic images by using optical coherence tomography (OCT), fundus cameras for photographing the fundi, and scanning laser ophthalmoscopes (SLOs) for obtaining images of the fundi by laser scanning using confocal optical systems.
In ophthalmic examinations using such apparatuses, alignment between an optical system and the subject's eye is important in terms of examination precision and accuracy. Examples of alignment generally include the operation (XY alignment) of aligning the subject's eye with the optical axis of an optical system and the operation (Z alignment) of adjusting the distance between the subject's eye and the optical system to a predetermined operating distance.
Some of such alignment techniques are designed to execute XY alignment and Z alignment based on pupil images captured by stereo cameras (patent literature 1 (Japanese Patent Laid-Open No. 2013-248376), patent literature 2 (Japanese Patent Laid-Open No. 2014-113385), and patent literature 3 (Japanese Patent Laid-Open No. 2014-124370)). Alignment using such a stereo camera covers a wide alignment range, requires no manual alignment, and allows automatic alignment in most cases.
In alignment using a conventional stereo camera, however, because the distance between a reference position on the apparatus and the pupil is set as a reference, the vertex of the cornea cannot be positioned due to individual differences in anterior chamber depth. Although patent literature 3 discloses a study on a method of correcting the depth position information of a cornea image using corneal refractive power, it is necessary to separately provide a mechanism for measuring the curvature of the cornea to obtain a corneal refractive power.
The stereo camera method as operating distance alignment means is influenced by individual differences in anterior chamber depth, corneal curvature, and the like, and hence cannot be used for devices requiring alignment with respect to the vertex of the cornea.